1. Field of the Invention
This invention relates generally to fluid dispensing devices employing gas-generating cells as a propulsion component.
2. State of the Art
Various devices have been utilized as fluid-dispensing apparatus, especially for liquid fluids, where the fluids are dispensed over an extended period of time at a predictable, substantially constant rate.
Battista in U.S. Pat. No. 3,115,280 disclosed a device which can be utilized to dispense fluids by generating H.sub.2 and O.sub.2 gases by electrochemically decomposing water at electrodes. Fluid contained in a flexible reservoir is dispensed as the generated gases pressurize an adjacent chamber in which the reservoir is contained except for an outlet through which the dispensing fluid leaves the device. The aqueous medium which is decomposed to form H.sub.2 and O.sub.2 gases surrounds the dispensing liquid reservoir.
Richter in U.S. Pat. No. 3,894,538 disclosed a similar device for dispensing a fluid. In this case, the electrochemically generated gas enters a separate chamber (gas chamber) which shares a flexible diaphragm wall with a liquid containing reservoir. As gas is generated, the liquid is dispensed. Richter suggests several means by which gas may be electrochemically generated including through the use of a cell utilizing an anode consisting of zinc, cadmium, or aluminum.
Orlitzky in U.S. Pat. No. 4,023,648 discloses a similar device which utilizes zinc or magnesium anodes in a cell to electrochemically generate hydrogen gas to pressurize a gas chamber separated from a fluid chamber by a "gas-proof diaphragm." Orlitzky claims that the device is constructed "so that it is almost impossible for any of the generated gas to escape."
Similarly, in U.S. Pat. No. 5,242,565, Winsel discloses a hydrogen generating galvanic cell which utilizes zinc anodes in an alkaline electrolyte to displace a fluid.
Bae et. al. in U.S. Pat. No. 5,354,264 discloses a similar device where water is electrochemically decomposed from an aqueous soaked hydrogel to form hydrogen and oxygen to pressurize a gas chamber with a flexible diaphragm shared by a fluid chamber, or the generated gas enters a chamber of a syringe separated from the liquid by a plunger or cylinder.
The devices described hereinabove are not designed for long shelf life, especially when they are mated to bladder type fluid delivery reservoirs. Moreover, the existing art has ignored the fact that the actual fluid delivery rate is a function of both the rate of gas generation and the rate of transport through the gas chamber walls and seals. This is especially true for slow rate devices.
The fluid dispensing devices described above all generate gas in amounts directly proportional to the electrical current passing through the device circuit; however, it has been discovered that the actual fluid delivery rate is a function of materials of construction which affect the rate of gas transport across the gas chamber walls and seals to and from the ambient air in addition to the rate of gas generation. These fluxes can be very significant when hydrogen is the primary gas generated. Typically the gas chamber outer shell of the devices described above is &lt;0.030 inches thickness, and the flexible diaphragm between the gas and liquid chamber is &lt;0.005 inches thickness. Syringe barrels typically are &lt;0.060 inches thickness. Since there is virtually no hydrogen in air, the gradient for permeation of hydrogen leaving the gas chamber is high. In addition, for plastics which are commonly utilized as materials for such devices, the permeation coefficient for hydrogen is higher than that for air. The ratio of hydrogen to air permeation coefficients at 25.degree. C. ranges from as low as 2.1 for cellophane to 93 for polypropylene. Thus permeation of hydrogen leaving the gas chamber always exceeds the permeation of air entering the chamber, resulting in a net flux of gas leaving the chamber. It has been discovered that the overall rate of liquid dispensed from the type of devices described above is a function of the materials utilized for construction, the area of the surfaces, and the material thicknesses, in addition to the gas generation rate. The effects of permeation are most evident when low pumping rates are desired because the effect of permeation is proportionally higher.
Conversely, many users of such devices are concerned about the presence of hydrogen since the gas can react exothermically in the presence of the oxygen in air if exposed to a spark. Thus, it may be desirable to permit the escape of hydrogen quickly and passively when the useful life of the device has ended.
One of the most important success criteria of fluid delivery devices is having adequate shelf-life; typically shelf life greater than two years is required. The prior art does not address this issue. Shelf life of prior art devices is short because of three issues. First is the loss of moisture from the gas generating cell due to permeation through the gas chamber shell or through the flexible diaphragm. Since most of the reactions which generate hydrogen involve the consumption of water, desiccation of the cells typically will have a negative impact resulting in lower performance or shorter than desirable life. Secondly, if the gas generators are the type which consume a metal, if oxygen is uncontrollably admitted to the cell, the metal will oxidize prematurely, and be spent when the device is to be activated. Third, if the gas generators are the type which consume a metal, hydrogen is generated to some degree prematurely. Corrosion inhibitors may be utilized to significantly reduce this effect; nevertheless, some hydrogen generation will occur if the active metal is in the presence of the aqueous solution, especially if the device is exposed to elevated temperature during storage. This hydrogen must be vented passively, otherwise the device will prematurely pressurize resulting in premature dispensing of the liquid, deformation of the device, or an undesirable burst of fluid delivery when the device is first activated. Thus, another object of this invention is to provide guidelines for selection of materials and design of the device which will be conducive to long shelf life.
Another concern of the users of fluid delivery devices when the device is of the type which electrochemically consume a metal to form hydrogen, is the delay before pumping occurs once the device is activated. This is because any oxygen which has diffused into the headspace between the gas generating cell and the flexible diaphragm must be consumed before hydrogen generation begins. It is also an object of this invention to disclose ways to minimize or avoid this start-up delay.
Another concern of the users of fluid delivery devices when the device is of the type which electrochemically consume a metal to form hydrogen is that typically in the prior art, the metals are amalgamated with mercury to reduce the amount of corrosion while being stored. Ultimate disposal of the device results in environmental problems since mercury is toxic and accumulates in the food chain. Another object of this invention is to disclose ways to avoid the need to amalgamate the electrochemically active metals without sacrificing performance.
Winsel in German Patent 3,602,214 discloses a chemical corrosion technique of generating hydrogen gas from a metal in the presence of an aqueous solution. The technique involves plating a second metal over the corroding metal. Similarly, hydrogen generation from chemical corrosion of a metal for fluid delivery is disclosed in German Patent 2,139,771 and Canadian Patent 961,420. Sancoff has disclosed in U.S. Pat. Nos. 5,398,850 and 5,398,851 storage stable devices utilized for dispensing fluids which are driven by carbon dioxide gas released when a material containing carbonates or bicarbonates is combined with an acid. Sancoff's devices have separate compartments for the reacting constituents to prevent them from reacting during storage, and a means to enable the combining of the active constituents at the time of activation. Such devices utilizing carbonates and bicarbonates have the tendency to not deliver at consistent rates without the utilization of pressure relief valves. The devices present herein are capable of providing nearly constant rate delivery without the added complexity of incorporating a pressure relief valve.